mail upgrade hangs

Hi, I'm sure this has happened to others, but there is not much posted since July.

I did the upgrade to Mountain Lion on three macs.

My 21 inch Mac had no problem upgrading the mail database as there was not much old email stored there.

My home system 27 inch iMac, has started the upgrade process for the mail database and after about 12 hours it's hung about 1/4 the way through.

Any ideas?

Should I reboot, or just let it run?

I have my mail data on my SSD, have a 2 TB spinning disk, and an external Lacie Thunderbolt RAID drive.

I can hear disk activity on the thunderbolt drives, but it's doing a backup.

Could TIME Machine be causing the mail hang?

Mike